We need to figure out how to make automatic payments while avoiding double-spending when the Zap Event disappears. It does happen (the relay used for the zap event goes off-line, or is cleaned up, etc). Since Zap Events are not spread in as many relays as possible, it is likely that a client might not see it and try to pay again.

Ideally, the subscription payment is made by the NWC service directly for 2 reasons: the service is always online (which means it won't miss a payment just because the user hasn't connected in a while) and the service knows when it already paid for that subscription better than any client would. The only issue is on how to avoid two NWC services paying the same subscription...